The invention pertains to the field of F.sub.2.sup.+ color center lasers.
The F.sub.2.sup.+ color center in alkali halide crystals has been shown to provide a nearly ideal gain mechanism for efficient, broadly tunable, optically pumped, CW and pulse lasers in the near infrared. This is because:
(1) The small Stokes shift for the infrared transition implies that the emission band of the F.sub.2.sup.+ color center has the same high oscillator strength (f.about.0.2) as has the absorption band; PA1 (2) There is strong evidence for a nearly 100 percent quantum efficiency; PA1 (3) Using the calculated energies of the H.sub.2.sup.+ molecular ion, the splitting between the lowest lying even parity excited state 2S.sub..sigma..sbsb.g and the 2P.sub..sigma..sbsb.u state can be predicted to be much too large to allow for self-absorption at the emission energy, and PA1 (4) Thus far there are no known color center species foreign to the F.sub.2.sup.+ that would absorb photons of the lower energy emission band other than one center type which is probably a variety of F.sub.3.sup.+ color center and is easily eliminated or otherwise avoided. PA1 (a) causes an essentially complete conversion of F.sub.2 to F.sub.2.sup.+ color centers, PA1 (b) obtains the conversion in the presence of an external electron trap density no greater than that of the original F.sub.2 color center population, and PA1 (c) is selective in such a manner that substantially no positively ionized species other than the F.sub.2.sup.+ color centers are produced. PA1 (1) incorporating, for external electron traps, a sufficient number of divalent metal ions, of a size that sets well in the crystal lattice, into an alkali halide crystal; PA1 (2) creating anion vacancies and F color centers in the alkali halide crystal when it is cooled sufficiently to prevent vacancy diffusion (such anion vacancies may be created by such mechanisms as radiation damage from electron beams, high intensity gamma rays, or high intensity X-ray sources); PA1 (3) warming the alkali halide crystal to room temperature for a short time to allow F and F.sub.2.sup.+ color centers to form therein; PA1 (4) cooling the crystal to laser-operating temperature; and PA1 (5) irradiating the alkali halide crystal with the appropriate radiation to provide the two-step photoionization.
The laser action does not seem to suffer from bleaching or aging effects during normal operation as contrasted by such effects in the organic dyes. Also, the required optical pump power at threshold is usually many times less than that required for the most efficient dye lasers.
The F.sub.2.sup.+ color center laser in alkali halides will provide coverage for laser outputs in the range 0.8.ltorsim..lambda..ltorsim.2 .mu.m. This region is of fundamental importance to molecular spectroscopy, pollution detection, fiber optic communications, and the physics of narrow-band-gap semiconductors.
However, the production of F.sub.2.sup.+ color center densities high enough for efficient use in lasers, especially in those whose cavity modes are tightly focused, has been a major source of difficulty.
F.sub.2 color centers are typically converted to F.sub.2.sup.+ color centers by subjecting the F.sub.2 color centers to ionizing radiation. The conversion is permanent if suitable electron traps have been provided for the excess electrons ejected from the F.sub.2 color centers. However, there is a need for an alternative to the ordinary (single-step) photoionization process for creating F.sub.2.sup.+ color centers. This need arises because the present methods of creating F.sub.2 or F.sub.2.sup.+ color centers results in densities which are considerably outnumbered by accompanying F color centers. The ground states of the F and F.sub.2 color centers lie approximately the same distance below the conduction band in the crystal and the photoabsorption cross-sections and ionization efficiencies of these two color centers are comparable in the single-step photoionization range. These conditions cause the fractional ionization of F.sub.2 color centers obtained using the single-step photoionization process to be low. The fractional ionization may be improved by adding external electron traps, but the density of such external traps must be larger than the sum of all ionizable species. Such large densities of traps are very hard to achieve in practice and even if achievable, they would lead to other undesirable effects such as obtaining a large density of F.sup.+ color centers. The F.sup.+ color centers, due to their mobility at temperatures in excess of .about.200.degree. K., would destabilize the admixture of color center populations. In particular, one would risk creating a variety of F.sub.3.sup.+ color centers which tend to absorb radiation in the region of the F.sub.2.sup.+ luminescence band.